I. Field of the Invention
This invention relates to a bridge plug which may be set by either wireline or mechanical means and, in particular, to a bridge plug made of cast iron which may be quickly drilled out of the casing.
II. Description of the Prior Art
Various bridge plug configurations have been utilized to seal a well casing but generally the most common are bridge plugs of the type having upper and lower anchor slips with an elastomeric packer element positioned about an inner mandrel therebetween. Bridge plugs may be set by either mechanical or wireline means to anchor the slips and compress the packer element into sealing engagement with the casing wall. Once set, the plug can be subject to extreme pressures and temperatures and the plug must be capable of withstanding these conditions without destruction of the seal formed by the packer element. At a later time it may become advantageous to remove the plug in order to continue working the well.
The simplest method of removing a bridge plug is to drill out the plug as the drill bit is run into the casing to further develop the well hole. However, it has been found that bridge plugs having even a minimal amount of steel are difficult to drill during removal and can damage the drill bit. The steel in the structural body of the plug is utilized to provide structural strength to set the tool particularly in the form of a shear stud which screws into the top of the plug. The disadvantage of this configuration is that when the stud is sheared to separate the setting tool from the plug, a small stub of steel remains in the plug which is difficult to drill out.
Past known bridge plugs have also been provided with means for maintaining packer compression independent of that provided by the slips. Such bridge plugs include ratchet mechanisms which maintain the packer in a compressed condition. In most bridge plugs the lock nut or ratchet mechanism is positioned either above the upper slip or inside the upper compression cone. The disadvantage of this design is that the packing element must pass over the threads on the mandrel required for the lock nut to ratchet over and lock into place. When the element passes over these threads, there is a good chance that the packer may be damaged thereby reducing its sealing capability.